strech animation faster / slower in parts
If you select your path and go to the path object data tab form the properties pane, scroll down to path animation, there's the property frames which you can animate (hover and press i) to increase or decrease the amount of frames required to traverse the path at any given time.
So for example, if the value is at 100 at first, but starting from frame 50 you want the movement to be slower until frame 100, then at frame 50 you would create a keyframe (still at value 100), at frame 100 you would create another keyframe with value 150 for example.